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                <h2 id="Flutter-Bloc实战总结"><a href="#Flutter-Bloc实战总结" class="headerlink" title="Flutter Bloc实战总结"></a>Flutter Bloc实战总结</h2><p>英文全称(business logic),用于处理业务逻辑,其内部实现主要是对Stream的输入和输出进行了封装,它的实现原理是利用RxDart(基于Stream封装)提供的<code>PublishSubject</code>和<code>BehivorSubject</code>实现了<code>Event</code>和<code>State之间的转换</code>, 与MVVM的分层设计非常相似， 利用Flutter提供的局部状态管理控件<code>InheritedWidget</code>来传递<code>Bloc</code>对象,涉及的类比较多,但基本都围绕着对上面这个对象的封装. 它的代码结构简单,风格统一,便于维护和bug定位。</p>
<h2 id="使用介绍"><a href="#使用介绍" class="headerlink" title="使用介绍"></a>使用介绍</h2><p><a target="_blank" rel="noopener" href="https://bloclibrary.dev/">https://bloclibrary.dev/</a></p>
<h2 id="工作流程"><a href="#工作流程" class="headerlink" title="工作流程"></a>工作流程</h2><ol>
<li>通过Provider负责提供Bloc对象,注册到当前的context中,通过InheritedWidget可以非常方便的获取它的先祖的bloc, MultiBlocProviders则负责将多个Provider嵌套在一起,让代码变得更好看一些。</li>
<li>Bloc接口来自service层(非UI事件)和widget(ui的事件),在bloc内部完成相应的逻辑并将新的state返回出去，可以重写父类的<code>transfromEvents</code>方法对Event进行 filter,debounce,switchMap操作。</li>
<li>通过继承HydrateBloc可以实现状态的实时缓存功能(这一部是可选的)</li>
<li>在Widget中通过嵌入BlocListener/BlocBuilder,根据之行的泛型自动完成对Bloc的订阅来实现widget的更新和bloc状态的的监听，其中BlocBuilder主要用来触发构建。 BlocListener则主要用来处理界面业务事件，如push/pop/showDialog</li>
</ol>
<p> 下面草图简要的说明了bloc(event/state)和其它模块之间的数据交互关系, 本质上就是接收Event,处理逻辑，生成State， 其它服务或者页面订阅state处理下一步逻辑。</p>
<p><img src="https://img-blog.csdnimg.cn/img_convert/2ac35f0bc229acf32a2ea03bce9e3fff.png" alt="截屏2020-09-30 上午6.26.53.png"></p>
<h2 id="主要类之间的关系"><a href="#主要类之间的关系" class="headerlink" title="主要类之间的关系"></a>主要类之间的关系</h2><p><img src="https://img-blog.csdnimg.cn/img_convert/dd519da5588f798e10e5b6e4ad4ccb4d.png" alt="FlutterBloc Classes.png"></p>
<h2 id="挂载到Element-Tree上的"><a href="#挂载到Element-Tree上的" class="headerlink" title="挂载到Element Tree上的"></a>挂载到Element Tree上的</h2><ol>
<li><p>首先自定义Bloc</p>
<p>自定义一个继承Bloc的类,并实现<code>mapEventToState</code>的方法逻辑，完成事件输入到数据输出之间的转换,</p>
</li>
<li><p>注册Bloc</p>
</li>
</ol>
<figure class="highlight dart"><table><tbody><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br></pre></td><td class="code"><pre><span class="line">BlocProvider&lt;YourCustomWidget&gt;(</span><br><span class="line">    create: (context) =&gt; MyCustomBloc(),</span><br><span class="line">    child: YourCustomWidget(),</span><br><span class="line">);</span><br></pre></td></tr></tbody></table></figure>

<ol start="3">
<li>这里需要用到<code>BlocProvider</code>来包装<code>Bloc</code>的构造方法和它的获取方法<br><code>BlocProvider</code>继承于<code>ValueDelegateWidget</code>,并实现了<code>SingleChildCloneableWidget</code>,包含一个<code>ValueStateDelegate</code>对象, ValueStateDelegate<code>主要是用于包装</code>Bloc<code>对象,用于控制是否更新或者销毁bloc , ValueDelegateWidget</code>继承于<code>DelegateWidget</code>,在其<code>State</code>事件<code>initState</code>,<code>didUpdateWidget</code>,<code>dispose</code>中对<code>ValueStateDelegate</code>进行去重,更新和销毁操作,这样就能间接作用于bloc上(这样减少了Bloc的方法,起到了一定的解耦合作用),StateDelegate分为2中,<code>BuilderStateDelegate</code>支持关闭<code>bloc</code>,而<code>ValueStateDelegate</code>不会自动关闭<code>bloc</code>,在使用中如果不是全局单例Bloc,则容易出现内存泄漏.</li>
</ol>
<p>4.<code>BlocProvider</code>在build时他会在内部创建一个<code>InheritedProvider</code>,它接受了一个<code>Bloc</code>作为值,由于<code>InheritedProvider</code>是继承于<code>InheritedWidget</code>,所以<code>Bloc</code>就可以通过<code>InhertedElement</code>间接的存储到当前的的Element中,它的<code>child</code>内部就能直接通过Context获取到.</p>
<ul>
<li><code>SingleChildCloneableWidget</code>这个类主要是定义了一个clone的协议，主要是为后面的<code>MutliBlocProvider</code>服务的,将单个child拷贝多分,设置到每个<code>BlocProvider</code>中,是框架内部的一个语法糖。</li>
</ul>
<ol start="5">
<li>在<code>BlocProvider</code>注册的过程中,可以看到它必须要又一个child,它自己本身是不需要在界面显示,只是作为一个数据的提供者,所有在它的之下的widget都能通过<code>BlocProvder.of&lt;BlocName&gt;(context)</code>获取到这个对应的Bloc。</li>
</ol>
<figure class="highlight dart"><table><tbody><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br></pre></td><td class="code"><pre><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">BlocProvider</span>&lt;<span class="title">T</span> <span class="keyword">extends</span> <span class="title">Bloc</span>&lt;<span class="title">dynamic</span>, <span class="title">dynamic</span>&gt;&gt;</span></span><br><span class="line"><span class="class">    <span class="keyword">extends</span> <span class="title">ValueDelegateWidget</span>&lt;<span class="title">T</span>&gt; <span class="keyword">implements</span> <span class="title">SingleChildCloneableWidget</span> </span>{ </span><br><span class="line">  <span class="keyword">final</span> Widget child;</span><br><span class="line"></span><br><span class="line">  <span class="comment">//BlocProvider销毁时bloc会被自动关闭</span></span><br><span class="line">  BlocProvider({ ...</span><br><span class="line">    <span class="meta">@required</span> ValueBuilder&lt;T&gt; create,<span class="comment">//在init时执行此方法创建bloc</span></span><br><span class="line">    Widget child,</span><br><span class="line">  })  </span><br><span class="line"></span><br><span class="line">  <span class="comment">//BlocProvider销毁时bloc不会自动关闭</span></span><br><span class="line">  BlocProvider.value({ ...</span><br><span class="line">    <span class="meta">@required</span> T value, <span class="comment">//bloc</span></span><br><span class="line">    Widget child, <span class="comment">//具体的页面</span></span><br><span class="line">  });</span><br><span class="line"></span><br><span class="line">  <span class="comment">//这里只是用`Provider`的类方法包装了一层,将通过InheritedElment获取方法`context.inheritFromWidgetOfExactType(type) as InheritedProvider&lt;T&gt;`</span></span><br><span class="line">  <span class="keyword">static</span> T of&lt;T <span class="keyword">extends</span> Bloc&lt;<span class="built_in">dynamic</span>, <span class="built_in">dynamic</span>&gt;&gt;(BuildContext context) { ...</span><br><span class="line">    </span><br><span class="line">  <span class="comment">//通过`InheritedProvider`存储bloc,提供provider获取的方法</span></span><br><span class="line">  <span class="meta">@override</span></span><br><span class="line">  Widget build(BuildContext context) {</span><br><span class="line">    <span class="keyword">return</span> InheritedProvider&lt;T&gt;(</span><br><span class="line">      value: delegate.value,</span><br><span class="line">      child: child,</span><br><span class="line">    );</span><br><span class="line">  }</span><br><span class="line">  </span><br><span class="line">  <span class="comment">//方便MutliBlocProvider使用</span></span><br><span class="line">  <span class="meta">@override</span></span><br><span class="line">  BlocProvider&lt;T&gt; cloneWithChild(Widget child) {</span><br><span class="line">    <span class="keyword">return</span> BlocProvider&lt;T&gt;._(</span><br><span class="line">      key: key,</span><br><span class="line">      delegate: delegate,</span><br><span class="line">      child: child,</span><br><span class="line">    );</span><br><span class="line">  }</span><br><span class="line">}</span><br></pre></td></tr></tbody></table></figure>

<p>简而言之就是将Bloc挂在到InheritedWidget上去, 在widget被移除时根据它Bloc的初始化方式可以选择关闭Bloc的订阅流事件或者继续保持流事件（对于全局单利类的bloc我们应该保持它的流事件不被关闭，而对于某个模块或页面使用的bloc应在推出模块或页面时关闭), 分别对应<code>BlocProvider.create</code>和<code>BlocProvider.value</code>两种创建方式。</p>
<h2 id="BlocBuilder的工作流程"><a href="#BlocBuilder的工作流程" class="headerlink" title="BlocBuilder的工作流程"></a>BlocBuilder的工作流程</h2><p>通过<code>BlocBuilder</code>代码实现可以看到,它在初始化的时候会自动捕获<code>assentor</code>的bloc,或者直接在初始化的时候传入一个bloc,通过监听内部的bloc状态,并调用setState来更新当前的Widget Tree,在<code>BlocBuilder</code>和它继承的<code>BlocBuilderBase</code>实现, 这里的条件主要是用来过滤Bloc的state,用于触发不同的<code>child widget</code>构建</p>
<figure class="highlight dart"><table><tbody><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br><span class="line">41</span><br><span class="line">42</span><br><span class="line">43</span><br><span class="line">44</span><br><span class="line">45</span><br><span class="line">46</span><br><span class="line">47</span><br><span class="line">48</span><br><span class="line">49</span><br><span class="line">50</span><br><span class="line">51</span><br><span class="line">52</span><br><span class="line">53</span><br><span class="line">54</span><br><span class="line">55</span><br><span class="line">56</span><br><span class="line">57</span><br><span class="line">58</span><br></pre></td><td class="code"><pre><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">BlocBuilder</span> .. <span class="keyword">extends</span> <span class="title">BlocBuilderBase</span> </span>{ ...</span><br><span class="line"> </span><br><span class="line">  <span class="keyword">final</span> BlocWidgetBuilder&lt;S&gt; builder;</span><br><span class="line"></span><br><span class="line">  <span class="comment">/// <span class="markdown">{@macro blocbuilder}</span></span></span><br><span class="line">  <span class="keyword">const</span> BlocBuilder({</span><br><span class="line">    Key key,</span><br><span class="line">    <span class="meta">@required</span> <span class="keyword">this</span>.builder,</span><br><span class="line">    B bloc,</span><br><span class="line">    BlocBuilderCondition&lt;S&gt; condition,)</span><br><span class="line"></span><br><span class="line">  <span class="meta">@override</span></span><br><span class="line">  Widget build(BuildContext context, S state) =&gt; builder(context, state);</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">_BlocBuilderBaseState</span>&lt;<span class="title">B</span> <span class="keyword">extends</span> <span class="title">Bloc</span>&lt;<span class="title">dynamic</span>, <span class="title">S</span>&gt;, <span class="title">S</span>&gt;</span></span><br><span class="line"><span class="class">    <span class="keyword">extends</span> <span class="title">State</span>&lt;<span class="title">BlocBuilderBase</span>&lt;<span class="title">B</span>, <span class="title">S</span>&gt;&gt; </span>{ ... </span><br><span class="line">  <span class="comment">//记录当前bloc的订阅,用于在销毁时释放订阅</span></span><br><span class="line">  StreamSubscription&lt;S&gt; _subscription;</span><br><span class="line">  <span class="comment">//状态值记录,用于回调给 api调用者过滤不同的 condition,决定当前的state是否需要触发新的构建</span></span><br><span class="line">  S _previousState;</span><br><span class="line">  S _state;</span><br><span class="line">  B _bloc;</span><br><span class="line"></span><br><span class="line">  <span class="meta">@override</span></span><br><span class="line">  <span class="keyword">void</span> initState() {</span><br><span class="line">    <span class="keyword">super</span>.initState();</span><br><span class="line">    <span class="comment">//可以看到bloc还可以直接重BlocBuilder传入,但这样就不支持BlocProvider便利获取了</span></span><br><span class="line">    _bloc = widget.bloc ?? BlocProvider.of&lt;B&gt;(context);</span><br><span class="line">     ...</span><br><span class="line">    _subscribe();</span><br><span class="line">  }</span><br><span class="line"></span><br><span class="line"> <span class="comment">//更新bloc的订阅</span></span><br><span class="line">  <span class="meta">@override</span></span><br><span class="line">  <span class="keyword">void</span> didUpdateWidget(BlocBuilderBase&lt;B, S&gt; oldWidget) { ...</span><br><span class="line">  </span><br><span class="line">  <span class="comment">//外部widget构建的回调方法</span></span><br><span class="line">  <span class="meta">@override</span></span><br><span class="line">  Widget build(BuildContext context) =&gt; widget.build(context, _state);</span><br><span class="line">  </span><br><span class="line">  <span class="comment">//释放bloc的订阅</span></span><br><span class="line">  <span class="meta">@override</span></span><br><span class="line">  <span class="keyword">void</span> dispose() {</span><br><span class="line">    _unsubscribe();</span><br><span class="line">    <span class="keyword">super</span>.dispose();</span><br><span class="line">  }</span><br><span class="line">  </span><br><span class="line">  <span class="comment">//监听bloc,并跳过intialState,因为在initial方法中已经拿到了</span></span><br><span class="line">  <span class="keyword">void</span> _subscribe() {</span><br><span class="line">    <span class="keyword">if</span> (_bloc != <span class="keyword">null</span>) {</span><br><span class="line">      _subscription = _bloc.skip(<span class="number">1</span>).listen((S state) {</span><br><span class="line">        <span class="keyword">if</span> (widget.condition?.call(_previousState, state) ?? <span class="keyword">true</span>) {</span><br><span class="line">          setState(() { <span class="comment">//很熟悉的setState</span></span><br><span class="line">            _state = state;</span><br><span class="line">            _previousState = state;</span><br><span class="line">            ...</span><br><span class="line">}</span><br></pre></td></tr></tbody></table></figure>

<h2 id="BlocListener的工作流程"><a href="#BlocListener的工作流程" class="headerlink" title="BlocListener的工作流程"></a>BlocListener的工作流程</h2><ol>
<li><p>与BlocBuilder类似,它在初始化的时候会自动捕获<code>assentor</code>的bloc,并在widget开始和更新时订阅事件.</p>
</li>
<li><p>这里在实际项目中会碰到一个坑, （当我们在BlocListener第一次mount）时并监听它的state事件进行页面跳转失效，这是由于Flutter Widget还没有完成首帧的渲染，所以push会失败。 在实际时使用中我们可以通过自定义一个StatefulWidget对其进行包装,并通过添加首帧渲染完成的回调事件再进行跳转操作。</p>
</li>
</ol>
<figure class="highlight dart"><table><tbody><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br><span class="line">41</span><br><span class="line">42</span><br><span class="line">43</span><br><span class="line">44</span><br><span class="line">45</span><br><span class="line">46</span><br><span class="line">47</span><br><span class="line">48</span><br><span class="line">49</span><br><span class="line">50</span><br><span class="line">51</span><br><span class="line">52</span><br><span class="line">53</span><br><span class="line">54</span><br></pre></td><td class="code"><pre><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">BlocListener</span>&lt;<span class="title">B</span> <span class="keyword">extends</span> <span class="title">Bloc</span>&lt;<span class="title">dynamic</span>, <span class="title">S</span>&gt;, <span class="title">S</span>&gt; <span class="keyword">extends</span> <span class="title">BlocListenerBase</span>&lt;<span class="title">B</span>, <span class="title">S</span>&gt;</span></span><br><span class="line"><span class="class">    <span class="title">with</span> <span class="title">SingleChildCloneableWidget</span> </span>{</span><br><span class="line"> </span><br><span class="line">  <span class="comment">//初始化BlocListener,指定需要过滤的条件和设置listener执行的逻辑</span></span><br><span class="line">  <span class="keyword">const</span> BlocListener({</span><br><span class="line">    Key key,</span><br><span class="line">    <span class="meta">@required</span> BlocWidgetListener&lt;S&gt; listener,</span><br><span class="line">    B bloc,</span><br><span class="line">    BlocListenerCondition&lt;S&gt; condition,</span><br><span class="line">    <span class="keyword">this</span>.child,</span><br><span class="line">  })  </span><br><span class="line">  <span class="comment">//拷贝child到当前的BlocListener中</span></span><br><span class="line">  <span class="meta">@override</span></span><br><span class="line">  BlocListener&lt;B, S&gt; cloneWithChild(Widget child) { ...</span><br><span class="line">  <span class="meta">@override</span></span><br><span class="line">  Widget build(BuildContext context) =&gt; child;</span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"><span class="comment">//Listener的逻辑实现</span></span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">_BlocListenerBaseState</span>&lt;<span class="title">B</span> <span class="keyword">extends</span> <span class="title">Bloc</span>&lt;<span class="title">dynamic</span>, <span class="title">S</span>&gt;, <span class="title">S</span>&gt;</span></span><br><span class="line"><span class="class">    <span class="keyword">extends</span> <span class="title">State</span>&lt;<span class="title">BlocListenerBase</span>&lt;<span class="title">B</span>, <span class="title">S</span>&gt;&gt; </span>{</span><br><span class="line">  StreamSubscription&lt;S&gt; _subscription;</span><br><span class="line">  S _previousState;</span><br><span class="line">  B _bloc;</span><br><span class="line"></span><br><span class="line">  <span class="meta">@override</span></span><br><span class="line">  <span class="keyword">void</span> initState() {</span><br><span class="line">    <span class="keyword">super</span>.initState();</span><br><span class="line">    <span class="comment">//可以自带一个bloc，但不支持其它子widget通过BlocProvider便利获取</span></span><br><span class="line">    _bloc = widget.bloc ?? BlocProvider.of&lt;B&gt;(context);</span><br><span class="line">    _previousState = _bloc?.state;</span><br><span class="line">    _subscribe();</span><br><span class="line">  }</span><br><span class="line">  </span><br><span class="line">  <span class="comment">//更新当前的bloc,重新订阅</span></span><br><span class="line">  <span class="meta">@override</span></span><br><span class="line">  <span class="keyword">void</span> didUpdateWidget(BlocListenerBase&lt;B, S&gt; oldWidget) { ...</span><br><span class="line">  <span class="comment">//构建child widget,这里只是by pass,没有做任何操作</span></span><br><span class="line">  <span class="meta">@override</span></span><br><span class="line">  Widget build(BuildContext context) =&gt; widget.build(context);</span><br><span class="line"></span><br><span class="line">  <span class="comment">//订阅bloc state </span></span><br><span class="line">  <span class="keyword">void</span> _subscribe() {</span><br><span class="line">    <span class="keyword">if</span> (_bloc != <span class="keyword">null</span>) {</span><br><span class="line">      _subscription = _bloc.skip(<span class="number">1</span>).listen((S state) {</span><br><span class="line">        <span class="keyword">if</span> (widget.condition?.call(_previousState, state) ?? <span class="keyword">true</span>) { <span class="comment">//condition的过滤条件在这里</span></span><br><span class="line">          widget.listener(context, state);</span><br><span class="line">          _previousState = state;</span><br><span class="line">        }</span><br><span class="line">      });</span><br><span class="line">    }</span><br><span class="line">  }</span><br><span class="line"></span><br><span class="line">  <span class="keyword">void</span> _unsubscribe() { ...</span><br></pre></td></tr></tbody></table></figure>

<h2 id="Bloc的辅助类"><a href="#Bloc的辅助类" class="headerlink" title="Bloc的辅助类"></a>Bloc的辅助类</h2><p> 为了便面地狱式的嵌套<code>BlocProvider/BlocListener</code>,作者创建了<code>MultiBlocListener</code>和<code>MultiBlocProvider</code>,他们属于<code>StatelessWidget</code>,主要是利用一个数组存储多个<code>BlocProvider</code>或<code>BlocListener</code>,结合<code>SingleChildCloneableWidget</code>协议的实现</p>
<figure class="highlight dart"><table><tbody><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br></pre></td><td class="code"><pre><span class="line">  Widget build(BuildContext context) {</span><br><span class="line">    <span class="keyword">return</span> MultiProvider(</span><br><span class="line">      providers: listeners,</span><br><span class="line">      child: child,</span><br><span class="line">    );</span><br><span class="line">  }</span><br><span class="line"></span><br><span class="line"><span class="comment">// SingleChildCloneableWidget 协议的实现</span></span><br><span class="line">    <span class="meta">@override</span></span><br><span class="line">  Widget build(BuildContext context) {</span><br><span class="line">    <span class="keyword">var</span> tree = child;</span><br><span class="line">    <span class="keyword">for</span> (<span class="keyword">final</span> provider <span class="keyword">in</span> providers.reversed) {</span><br><span class="line">      tree = provider.cloneWithChild(tree);</span><br><span class="line">    }</span><br><span class="line">    <span class="keyword">return</span> tree;</span><br><span class="line">  }</span><br><span class="line"></span><br><span class="line">  <span class="meta">@override</span></span><br><span class="line">  MultiProvider cloneWithChild(Widget child) {</span><br><span class="line">    <span class="keyword">return</span> MultiProvider(</span><br><span class="line">      key: key,</span><br><span class="line">      providers: providers,</span><br><span class="line">      child: child,</span><br><span class="line">    );</span><br><span class="line">  }</span><br></pre></td></tr></tbody></table></figure>


<h2 id="Bloc数据缓存"><a href="#Bloc数据缓存" class="headerlink" title="Bloc数据缓存"></a>Bloc数据缓存</h2><p>主要是通过构建中间基类<code>HydratedBloc</code>,对Bloc的initialState和translation事件进行传递并缓存数据到storage.可以参考<code>HydratedBloc</code>实现,以Bloc官方提供的<code>HydratedBloc</code>为例,它主要包括<code>HydratedBloc</code>,<code>Your CustomBloc</code>和<code>Bloc</code>之间插入的基本,用于管理Bloc数据的缓存.<code>HydratedBlocDelegate</code>它继承了<code>BlocDelegate</code>,用于替换Bloc框架内部默认的BlocDelegate,拦截<code>Bloc</code>的<code>Translation</code>中的两个State并存储其每次变化的状态到缓存中去.</p>
<p>关键部分的代码实现:</p>
<figure class="highlight dart"><table><tbody><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br><span class="line">16</span><br><span class="line">17</span><br><span class="line">18</span><br><span class="line">19</span><br><span class="line">20</span><br><span class="line">21</span><br><span class="line">22</span><br><span class="line">23</span><br><span class="line">24</span><br><span class="line">25</span><br><span class="line">26</span><br><span class="line">27</span><br><span class="line">28</span><br><span class="line">29</span><br><span class="line">30</span><br><span class="line">31</span><br><span class="line">32</span><br><span class="line">33</span><br><span class="line">34</span><br><span class="line">35</span><br><span class="line">36</span><br><span class="line">37</span><br><span class="line">38</span><br><span class="line">39</span><br><span class="line">40</span><br><span class="line">41</span><br><span class="line">42</span><br><span class="line">43</span><br><span class="line">44</span><br><span class="line">45</span><br><span class="line">46</span><br><span class="line">47</span><br><span class="line">48</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">abstract</span> <span class="class"><span class="keyword">class</span> <span class="title">HydratedBloc</span>&lt;<span class="title">Event</span>, <span class="title">State</span>&gt; <span class="keyword">extends</span> <span class="title">Bloc</span>&lt;<span class="title">Event</span>, <span class="title">State</span>&gt; </span>{</span><br><span class="line">  <span class="comment">//内置一个默认的Storage,</span></span><br><span class="line">  <span class="comment">//BlocSupervisor.delegate这个是改写之后的 BlocDelegate</span></span><br><span class="line">  <span class="keyword">final</span> HydratedStorage _storage =</span><br><span class="line">      (BlocSupervisor.delegate <span class="keyword">as</span> HydratedBlocDelegate).storage;</span><br><span class="line">  </span><br><span class="line">  <span class="comment">//初始化时从缓存获取数据,并在子类自定义的Bloc中实现这个三个方法,</span></span><br><span class="line">  <span class="meta">@mustCallSuper</span></span><br><span class="line">  <span class="meta">@override</span></span><br><span class="line">  State <span class="keyword">get</span> initialState { ...</span><br><span class="line">  State fromJson(<span class="built_in">Map</span>&lt;<span class="built_in">String</span>, <span class="built_in">dynamic</span>&gt; json);</span><br><span class="line">  <span class="built_in">Map</span>&lt;<span class="built_in">String</span>, <span class="built_in">dynamic</span>&gt; toJson(State state);</span><br><span class="line">  </span><br><span class="line">  <span class="comment">//HydratedBloc内部使用,用于提供BlocState的key</span></span><br><span class="line">  <span class="built_in">String</span> <span class="keyword">get</span> id =&gt; <span class="string">''</span>; <span class="comment">//可以定义扩展缓存的key名字,用于区分不同的业务逻辑数据,一般公共的bloc逻辑类可以采用此类方法处理</span></span><br><span class="line">  Future&lt;<span class="keyword">void</span>&gt; clear() =&gt; _storage.delete(<span class="string">'<span class="subst">${runtimeType.toString()}</span><span class="subst">$id</span>'</span>); </span><br><span class="line">}</span><br><span class="line"></span><br><span class="line"><span class="comment">//Bloc state拦截,和保存</span></span><br><span class="line"></span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">HydratedBlocDelegate</span> <span class="keyword">extends</span> <span class="title">BlocDelegate</span> </span>{ ...</span><br><span class="line">  <span class="keyword">final</span> HydratedStorage storage; </span><br><span class="line">  <span class="keyword">static</span> Future&lt;HydratedBlocDelegate&gt; build({</span><br><span class="line">    Directory storageDirectory,</span><br><span class="line">  }) <span class="keyword">async</span> {</span><br><span class="line">    <span class="keyword">return</span> HydratedBlocDelegate(</span><br><span class="line">      <span class="keyword">await</span> HydratedBlocStorage.getInstance(storageDirectory: storageDirectory),</span><br><span class="line">    );</span><br><span class="line">  }</span><br><span class="line"></span><br><span class="line">  <span class="comment">/// <span class="markdown">{@macro hydratedblocdelegate}</span></span></span><br><span class="line">  HydratedBlocDelegate(<span class="keyword">this</span>.storage);</span><br><span class="line">  </span><br><span class="line">  <span class="comment">//关键步骤</span></span><br><span class="line">  <span class="meta">@override</span></span><br><span class="line">  <span class="keyword">void</span> onTransition(Bloc bloc, Transition transition) { </span><br><span class="line">        storage.write(<span class="string">'<span class="subst">${bloc.runtimeType.toString()}</span><span class="subst">${bloc.id}</span>'</span>,</span><br><span class="line">          json.encode(stateJson)</span><br><span class="line"></span><br><span class="line"></span><br><span class="line"><span class="comment">//这个类逻辑单一,只负责文件存储,提供一个存储的Storage给上面的`HydratedBlocDelegate`</span></span><br><span class="line"><span class="class"><span class="keyword">class</span> <span class="title">HydratedBlocStorage</span> <span class="keyword">implements</span> <span class="title">HydratedStorage</span> </span>{...</span><br><span class="line">   <span class="keyword">static</span> Future&lt;HydratedBlocStorage&gt; getInstance({</span><br><span class="line">    Directory storageDirectory,</span><br><span class="line">   })</span><br><span class="line">   Future&lt;<span class="keyword">void</span>&gt; delete(<span class="built_in">String</span> key) <span class="keyword">async</span> {</span><br><span class="line">   Future&lt;<span class="keyword">void</span>&gt; delete(<span class="built_in">String</span> key) <span class="keyword">async</span> {</span><br><span class="line">   Future&lt;<span class="keyword">void</span>&gt; clear() <span class="keyword">async</span> {</span><br></pre></td></tr></tbody></table></figure>

<p>通过上面State部分的代码可以看出,此类bloc的设计主要是对Bloc的initialState和translation拦击来自动保存数据,默认提供了一个key,如果有多个不同的state则需使用多个state.这种写法其实对性能有一定影响。因为它是基于状态值实施存储的,所以这就需要我们做一些额外的优化。 </p>
<ul>
<li><p>如果项目足够庞大,在初始化时则会有大量的bloc缓存加载,这样势必会加重Flutter I/O读取的速度,拖慢启动速度,而且不同的bloc之间有依赖关系,我们还需要保证他们的初始化顺序完全同步,这就要求我们从业务上尽可能的对bloc的依赖层级进行解耦,分不同的优先等级分批初始化;</p>
</li>
<li><p>另外尽量不要采用多个State状态缓存,不管是从代码管理,状态传值都会显得非常长不便;</p>
</li>
<li><p>此外为了缓解项目类同时过多的translation并发读写,可以对其进行扩展绑定bloc的dispose事件,标记state之后再慢慢缓存(以内存空间的部分缓存来解决磁盘空间的频繁缓存)。</p>
</li>
</ul>
<h2 id="Bloc获取网络数据显示到界面"><a href="#Bloc获取网络数据显示到界面" class="headerlink" title="Bloc获取网络数据显示到界面"></a>Bloc获取网络数据显示到界面</h2><ul>
<li>提前注册<code>Repository</code>,注册方式同<code>Bloc</code>类似,也是通过BlocProvider提供, 然后将<code>Repository</code>赋值给bloc,这样在bloc内部的<code>mapEventToState</code>的方法中就可以根据不同的event时间去访问网络请求了。</li>
</ul>
<h2 id="Bloc使用注意事项"><a href="#Bloc使用注意事项" class="headerlink" title="Bloc使用注意事项"></a>Bloc使用注意事项</h2><ol>
<li>类型推导: Bloc中所有的类型都是采用泛型推导的,这就要求我们在使用的时候不要忘记了指定我们它的类型. BlocListener和BlocBuilder都是根据类型推导然后自动获取最近的bloc,如果没有指定类型则会报错.</li>
</ol>
<figure class="highlight dart"><table><tbody><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br></pre></td><td class="code"><pre><span class="line">BlocProvider&lt;CustomBloc&gt;.of(context);</span><br><span class="line"></span><br><span class="line">BlocBuilder&lt;CustomBloc,CustomBlocState&gt;(builder: (context, state) { ...</span><br><span class="line"></span><br><span class="line">BlocListener&lt;CustomBloc,CustomBlocState&gt;(listener: (preState, currentState){ ...</span><br></pre></td></tr></tbody></table></figure>

<ol start="2">
<li><p>Bloc的作用阈: BlocProvider作为Bloc载体,Bloc数据的存储会最终通过InheritedWidget存储在它当前所注册的作用阈中,如果我们在不同的作用域下注册Bloc,那么它将会被低级的作用域覆盖,因为此时WidgetTree中插入了2个不同层级的<code>InheritedWidget&lt;CustomBloc&gt;</code>,<br>如果想让他们共享统一个bloc,就必须要保证<code>InheritedWidget</code>所关联的Bloc是同一个value.根据前面对Bloc创建提到的2个类可以看出,Bloc创建分2中方法:  当Bloc所关联的BlocProvider widget被移除时Bloc的也将会被销毁.<br>这里举2个常见例子：</p>
<ul>
<li>比如我们在A页面注册了一个Bloc传递给B页面,如果调用了pushReplacement就会导致A页面释放,对应的Bloc就会跟这失效。 </li>
<li>当我们在使用StreamBuilder时，通过会根据不同的case创建不同的widget,如果Widget Tree变动, 而我们所使用的Bloc又是来自它的先祖通过BlocProvider.create方式创建的，那么这个bloc就会被dispose掉.  </li>
</ul>
</li>
<li><p>由于Bloc的State状态变化会触发界面刷新,因此需要合理拆分bloc的颗粒度,尽可能BlocBuilder的子树更小，过滤无需刷新界面的state.  对于小部件频繁变动的Widget可以采用statefulWidget进行封装， 对应页面局部变化频繁的模块可以单独拆分一个bloc进行监听(如Form表格)，</p>
</li>
<li><p>为了让页面的依赖关系更加清晰,代码的可独性更好, 一般页面通过如下方式搭建基本框架<br>  BlocProvider/MultiBlocProvider -&gt; BlocListener/MultiBlocListener -&gt; BlocBuilder -&gt; child</p>
</li>
<li><p>为了让项目各模块之间耦合性更低,尽量避免将一各模块的Bloc传入另一各模块的中使用，不同模块的之间的数据交互采用 repository(缓存传递数据), 对于多个模块需要实时联动的Bloc提取到公共父Widget的作用阈中。</p>
</li>
<li><p>通过创建一个中心bloc(类似EventBus),并通过泛型约束不同模块之间的数据出入参,然后注册不同的模块的订阅。（此方式会有一定的维护成本, 如果项目不是特别复杂采用方案5即可）</p>
</li>
<li><p>减少Bloc与Bloc之间的相互引用,尽量通过Repository和缓存获取数据，实时监听的情况除外.</p>
</li>
</ol>
<h2 id="总结"><a href="#总结" class="headerlink" title="总结"></a>总结</h2><p>Bloc框架目前已有5.6k左右,这种简单的固定格式的写法非常适合快速构建项目架构,由于它是基于stream来实现的,所以使用中要特别注意订阅的销毁,避免出现内存的泄漏,和冗余的订阅。它的数据传递利用了InheritedWidget的局部状态刷新来完成层,相比使用常规的<code>stateState</code>来刷新获取数据会更搞效率,这一点上也基本上参照了Flutter官方的设计,比如<code>ThemeData</code>,<code>Localization</code>,<code>MediaQuery</code>等全局数据的获取.它局部Widget构建<code>BlocBuilder</code>依然采用的setState方式构建,因此我们需要尽量避免在<code>BlocBuilder</code>过多的业务逻辑处理,和不必要widget渲染,尽量把静态的widget移出去,业务逻辑我们可以采用<code>BlocListener</code>对<code>BlocBuilder</code>包裹,来优化代码结构,和减少BlocBuilder的次数.</p>
<p>总的来说Bloc使用还是挺方便的, 相比较fish-redux也比较易于上手，不论从代码规范和可读性来讲都是挺不错的, 但它也有自己一些缺点, 如前面提到的 BlocListener初始化时的首次回调失效问题， 随着业务的增加Bloc树会越来越庞大(需要我们拆分一些公共类进行状态集中订阅和管理,有一定的维护成本)。 增对复杂的界面实时刷新，如多个功能类似的cell之间数据联动交互,会需要创建大量的bloc，需要考虑bloc的重建和循环利用， 对于复杂的动画交互效果不大适合采用bloc, 动画状态更新频繁，持续的传递bloc事件会有一定的性能开销, 但这些问题都是可以客户</p>

                
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